Polyurethane dispersions find many uses in industry. For example, polyurethane dispersions may be used to coat wood, plastic, metal, glass, fibers, textiles, leather, stone, concrete, ceramic or composite and other substrates to provide protection against mechanical, chemical and/or environmental effects. Polyurethane dispersions may also be used for adhesives, sealants, inks and cosmetics and the like.
Polyurethane coatings formed from polyurethane dispersions may be used to provide scratch, abrasion and wear resistance; UV protection; corrosion resistance; glossy or flat appearance; chemical and stain resistance; hydrolytic resistance; flame retardancy; anti-microbial activity; electrical conduction or insulation; barrier or permeability to gasses; adhesion; haptic effects such as soft touch; easy cleaning and anti-fingerprint.
Polyurethane dispersions are typically produced by first forming a polyurethane pre-polymer, which comprises terminal groups, such as isocyanate (NCO) groups, which can undergo subsequent chain extension reactions. To facilitate the formation of the dispersion, the polyurethane pre-polymer is usually diluted with an organic solvent before dispersion in water to lower the viscosity of the pre-polymer. The pre-polymer can undergo a chain-extension reaction prior to or after dispersion in water to increase the length of the polymer chain and/or add additional functionality to the polyurethane.
The use of organic solvents may be undesirable due to volatility, flammability and the difficulty in removing the organic solvents from the polyurethane dispersions. There is also increasing pressure for industrial processes to minimize the use of volatile organic compounds (VOCs). Many VOCs have been linked to environmental issues and may be hazardous to workers exposed to them.
The organic solvent is usually removed after dispersion in water. Distillation is a typical method for removing the organic solvent, but other methods may also be used. Acetone is a common solvent used in the preparation of polyurethane dispersions. Due to cost, acetone is often recycled for repeated use. However, wet acetone, i.e., acetone containing water, cannot be reused in polyurethane production processes. The separation of acetone in the polyurethane production adds cost, complexity and time. Due to the difficulty of removal, at least some acetone is expected to remain in the polyurethane dispersion.
U.S. Patent Application Publication No. 2012/0259065 discloses a process for preparing isocyanate-terminated pre-polymers with low viscosity. The pre-polymers are formed using a specific reaction sequence in which a reaction of NCO-functionalized compounds with OH-functionalized compounds initially takes place in the absence of acid carrying compounds. The isocyanate-terminated pre-polymer is formed in acetone solvent, which is later removed by distillation.
WO 2012/126911 discloses a process for forming polyurethane dispersions which contain polyurethane pre-polymers having a low enough viscosity such that they can be dispersed without the use of solvents or other diluents. However, the process uses hydroxyl terminal groups rather than isocyanate terminal groups, which limit the functionality of the polyurethanes. The pre-polymers are also not acrylated and cannot be UV curable.
Further information is available in U.S. Pat. No. 5,596,065 and publication WO 2012/089538 A1.